Method of propulsion using a monopropellant fuel containing organic sulfonium compounds



United States Patent ()fiice 3,Z3,569 Patented Mar. 6, 1362 This invention relates to monopropellant compositions suitable for use in rocket motors, ram-jets, pulse-jets and the like. In a further aspect, this invention relates to a method of operating such motors.

Rocket motors are operated by burning "a mixture of fuel and oxidant in a combustion chamber thereof and causing the resulting gases to be expel-led through a nozzle at high velocity. Liquid propellants are preferred over solid propellants where it is necessary to vary thrust during flight. Liquidpropellants can be classified as bipropellants and monopropellants, and the latter can be either a single compound or mixtures of compounds. Monopropellant systems are advantageous in that they require only one tank, one pump, one nozzle, one fuel line, one set of controls, etc. Furthermore, no mixing or proportioning system is required.

The principal elements of a rocket motor utilizing a liquid fuel comprise a combustion chamber, exhaust nozzle, an injection system, and propellant control valves. The propellant gases are produced in the combustion chamber at pressures governed by the chemical characteristics of the propellant, its rate of consumption, and the cross-sectional area of the nozzle throat. The gases are ejected into the atmosphere through the nozzle with supersonic velocity. The function of the nozzle is to convert the pressure of the propellant gases into kinetic energy. The reaction of the discharge of the propellant gases constitute the thrust developed by the rocket motor.

The following are objects of this invention.

An object of this invention is to provide new monopropellant composition. A further object of this invention is to provide a method for operating rocket motors. Other aspects, objects and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

In accordance with the invention there are provided new monopropell-ant compositions which are suitable for use according to the method of the invention in rocket motors and the like. Broadly speaking, the invention comprises a mixture of an organic sulfonium compound and a suitable oxidant as a monopropellant composition and the use of said composition .as a propellant in a rocket motor or the like.

Thus according to the invention there is provided a m-o'nopropellant composition comprising a mixture of 1) an oxidant selected from the group consisting of nitric acid containing at least about 70 weight percent HNO and mixtures of said nitric acid with perchloric acid wherein said mixtures contain up -to about 50 weight percent HClO and not more than about 30 Weight percent water, and (2) at least one organic sulfonium compound characterized by a formula selected from the group consisting and wherein: each R is selected from the group consisting of alkyl and cycloalkyl groups containing from 1 to 10 carbon atoms; R is selected from the group consisting of alkylene radicals containing from 1 to 8 carbon atoms; X is an anion selected from the group consisting of nitrate, perchlorate, sulfate, hydrogen sulfate, phosphate, hydrogen phosphate, and dihydrogen phosphate anions; a and b are each integers of from 1 to 3, and the product of.-a-

multiplied by the number of sulfur atoms is equal to the product of b multiplied by the valence z of said anion X;'

the total number of carbon atoms in the molecule is from 3 to 12; and wherein the ratio of said sulfonium compound to said oxidant is within the range of 0.75 to 1.25 times that of the stoichiometric amount.

Preferably, the number of carbon atoms in said sulfonium compounds is from 3 to 7. The most preferred sulfonium compounds are the nitrate and perchlorate salts of the above Formula I wherein the R groups are alkyl groups containing from 1 to 4 carbon atoms, and the nitrate and perchlorate salts of the above Formula II wherein the R groups are alkylene groups containing from 2 to 6 carbon atoms.

Examples or organic sulfonium compounds which are useful in the practice of the invention include, among others, the following:

Trimethylsulfonium nitrate Triethylsulfonium nitrate Triethylsulfoniurn perchlorate Tri-n-propylsu-lfonium nitrate Tri-n-propylsulfoniurn perchlorate Tri-n-butylsulfonium nitrate Triisopropylsulfonium nitrate Dimethylethylsulfonium sulfate Diet-hylmethylsulfonium nitrate Ethyldimethylsulfonium nitrate Ethyldimethylsulfonium hydrogen sulfate Ethyldibutylsulfonium nitrate Ethyldibutylsulfoniurn phosphate Dimethyloctylsulfonium nitrate Methyldipentylsulfonium hydrogen phosphate Ethyldipentylsulfonium nitrate Ethyldipentylsulfonium dihydrogen phosphate Hexylmethylpentylsulfonium nitrate Decyldimethylsulfonium nitrate Cyclohexyldimethylsulfonium nitrate Dicyclopentylmethylsulfonium nitrate Dimethylcyclopentylsulfonium nitrate Diethylcyclohexylsulfonium nitrate Dimethylcyclooctylsulfonium nitrate 2-cyclohexylpropyl ethylmethylsulfoniurn nitrate Cyclopentylmethyl) dimethylsulfoninrn nitrate S,S,S,S-tetnamethylmethane-1,1-disulfonium dinitrate S,S,S,S'-tetramethyletha ne-1,2-disulfonium dinitrate S,S-diethyl-S',S'-'dimethylethane-l,2-disulfonium dinitrate S,S,S,S'-tetraethylpropane-l,2-disulfonium dinitrate S,S,S',S'-tetraethylbutane-1,4-disulfonium dinitrate S,S,S',S-tetramethylhexane-1,6-disulfoniurn dinitrate S,S,S,S'-tetramethyl-l-ethylpropane-1,3-disulfonium dinitrate S,S,S',S'-tetraethylpropane-1,3-disulfoniurn dinitrate S,S,S,S'-tetramethylbutane-1,4-disulfonium dinitrate S,S,S,S-tetrarnethyloctane-1,8-disulfonium dinitrate S,S,S',S-tetramethylpropane-1,3-disulfoniurn diperchlorate S,S,S',S-tetraethylbutane-1,4-disulfoniurn diperchloratc For each of the above named nitrates or other compounds there can be prepared and used the correspondmg compounds having as the anionic component one of the other above named inorganic anions.

The organic sulfonium compounds described above are, oxygen deficient and consequently the monopropellant.

fuel compositions of the invention require an oxidant. Nitric acid is the presently preferred oxidant for use in the practice of the invention. Since water tends to retard combustion of the acid with the fuel, the nitric acid is preferably substantially free of water. Thus, the presently most preferred oxidant is anhydrous nitric acid. However, other more dilute nitric acids can be used in the practice of the invention. White fuming nitric acids and red fuming nitric acids of varying concentrations are available commercially, and all are useful in the practice of the invent-ion. White fuming nitric acid usually contains about 90 to 99 weight percent HNO from to 2 weight percent N0 and up to about weight percent water. Red fuming nitric acid usually contains about 70 to 90 weight percent HNO from 2 to 25 weight percent NO and up to about 10 weight percent water. Of course, mixtures of the above described acids can be employed to give an acid having any intermediate composition, and all are useful in the practice of this invention. Thus, it has been found that nitric acids of all types containing at least about 70 weight percent HNO are useful as an oxidant in the practice of the invention.

In addition, it is within the scope of the invention to use mixtures of said nitric acid with perchloric acid wherein said mixtures contain up to about 50 weight percent HClO as an oxidant in the practice of the invention. Said mixtures preferably do not contain more than about 30 weight percent water.

The monopropellants used in the present invention will be preferred near stoichiometric mixtures of oxidant and at least one of said organic sulfonium compounds. The ratio of fuel component (sulfonium compound) to oxidant can be in the range of 0.75 to 1.25 times that of the stoichiometric amount. In the practice of the invention said sulfonium compounds are commonly used in amounts of about 19 to about 53 weight percent of the mixture of oxidant and organic sulfone. A slightly fuel-rich mixture is usually required to :give an optimum rocket motor performance. Fuel rich compositions are also preferred for many gas generator applications since the resulting hot gases are less corrosive to metals. As used herein, stoichiometric ratio is that ratio of fuel to oxidant calculated by assuming complete combustion of the sulfur, hydrogen, and carbon in the organic cation of the fuel to S0 H 0, and CO respectively; and conversion of the nitrogen, chlorine, sulfur, or phosphorus in the anionic component of the fuel to N HCl, S0 and P 0 respectively.

The normally preferred procedure for preparing the monopropellants of the invention is to admix the organic sulfonium compound, prepared by any suitable method, with nitric acid or other suitable oxidant in the desired ratio at some time prior to use. -It is generally preferred to add the organic sulfone to the acid oxidant at temperatures below about 50 C., e.g., O to 30 C., with good agitation. The length of storage prior to use will depend upon the storage stability of the particular monopropellant composition being employed, said storage stability being determined as shown hereinafter.

The organic sulfonium compound-nitric acid monopropellants of the present invention can be conveniently ignited by contacting a stream of the monopropellant with a stream of a hypergol such as pyrrole. Any mate rial which is hypergolic when mixed with nitric acid can be used. Other materials hypergolic with nitric acid such as N,N,N',N'-tetramethylpropane-1,3-diamine; N,N,N, N'-tetramethylpropene-1,3-diamine; furfuryl alcohol; ethylenediamine; etc., can also be used to ignite said monopropellants. These hypergols are simultaneously injected into the combustion chamber with the monopropellant to ignite said monopropellant. After the monopropellant is ignited, the flow of hypergol is stopped. A temperaturesensitive element, a. time mechanism or other means can be used to terminate the flow of the hypergol. The mono- 4 propellant compositions of the present invention can also be ignited by other means such as, for example, by an electric igniter.

The organic sulfonium compounds used in the practice of the invention can be prepared by any of a number of suitable methods well known to those skilled in the art. For example, trialkyl sulfonium nitrates can be prepared by reacting dialkyl sulfides with alkyl halides. The resulting sulfonium halide can then be reacted with silver nitrate to precipitate silver halide and to free the sulfonium nitrate. The trialkyl sulfonium compounds can also be prepared from the corresponding hydroxides.

The disulfonium compounds can be prepared by methods comparable to those described above for the monosulfonium compounds. As an example, a dialkyl disulfide, such as S,S-diethylpropane-l,3-disulfide, is reacted with an alkyl halide, such as ethyl chloride. The resulting S,S,S,S'-tetraalkyl disulfonium halide is reacted with silver nitrate to free the disulfonium nitrate. For the specific compounds named, the product is S,S,S',S' tetraethylpropane-l,3-disulfonium dinitrate. When moist silver oxide is reacted wtih the sulfonium halide the corresponding sulfonium hydroxide is formed. The sulfonium hydroxide can be neutralized with the desired acid to form the corresponding salt, e.g., nitric acid forms the nitrate, perchloric acid forms the perchlorate, etc.

The following example will serve to further illustrate the invention.

EXAMPLE Two monopropellant solutions A and B were prepared by dissolving sulfonium compounds in essentially anhydrous nitric acid in the amounts shown in Table I below. Typical batches of said acid analyzed at least 99.8 weight percent HNO and contained less than 0.2 weight percent oxides of nitrogen. In propellant A the monosulfonium compound used was trimethylsulfonium nitrate. In propellant B the disulfonium compound used was S,S,S', S'-tetramethylpropane-1,3-disulfonium dinitrate. Results of various tests on said monopropellants are given in Table I ibelow:

Maximum pressure reached during test was about 35 p.s.i.g. which was reached in about 48 hours. Thereafter the pressure remained essentially constant.

b Maximum pressure reached during test was 50 p.s.i.g. which was reached after about 18 days. Thereafter the pressure remained essentially constant.

The above results illustrate the good storage stability of the monopropellant composition. The storage stability tests were terminated after the indicated number of hours; therefore, the storage stability values are listed as being greater than 763 hours and 765 hours for the monosulfonium and the disulfonium compounds, respectively. The shock sensitivity, which is an inverse function of the card gap range is entirely adequate. The viscosity determinations show that the solution can be pumped at temperatures at least as low as 40 F., and the freezing point is below -65 F.

The procedures used in carrying out the tests in the above example are outlined below.

prevented detonation.

The procedure followed in determining the storage stability can be summarized as follows. A small glass tube, constructed from A-inch (I.D.) glass pipe, which is able to withstand pressures greater than 1000 p.s.i., is filled about two-thirds full (about 6 ml.) with the monopropellant to be tested. Ihis tube is fitted with a safety head containing a blowout disk which will rupture at about 200 p.s.i. pressure. Said small glass bomb is then placed in a constant temperature bath, containing cold water, and is connected to a pressure recorder and to a supply of compressed nitrogen gas. The pressure in said bomb is raised to 110 p.s.i. with nitrogen to check the system for leaks after which the pressure in said bomb is reduced to 20 p.s.i. The temperature in the constant temperature bath, which can be regulated to maintain a temperature of 200 F., is increased and the time at which a temperature of 200 F. 'is reached is taken as the start of the test. The test is terminated when the pressure in said bomb exceeds 100 p.s.i. or when the rupture disk is ruptured (the pressure rise is often rapid after 100 p.s.i. is reached). The storage life of the propellant at 200 F. is recorded as the time necessary for the pressure in said bomb to increase from 20 to 100 p.s.i.

The shock sensitivity of the monopropellant was determined by the American Rocket Societys Recommended Card Gap Test No. 1, Committee on Monopropellant Test Methods, July 12, 1955. Basically said test consists of placing a 40 ml. sample of the monopropellant above a 50 gram tetryl booster charge and determining the number of 0.01 inch thick cellulose acetate disks (cards) which must be inserted between said booster and the monopropellant to prevent detonation of said monopropellant sample. The following minor modifications were made in the procedure.

Instead of using the prescribed steel cup for holding the monopropellant sample, the tests were run using a cup made of Pyrex glass pipe. The dimensions of the glass cup corresponded closely with those of the recommended steel cup:

The data for the card gap test are herein recorded as a range from the highest number of cards which did not prevent detonation to the lowest number of cards which The shock stability is inversely related to the card gap range.

The viscosities were determined with a Fenske viscometer following the procedure substantially as stated in ASTM test D445.

The density determinations were made with a Westphal balance.

The approximate freezing points were determined by attempting to cool the solution to the temperature at which crystal separation began. The tests were determined at a temperature above the freezing point in view of the low value.

Since many possible embodiments may be made of this invention without departing from the scope thereof, it is to be understood that all matter herein set forth is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. A monopropellant composition consisting essentially of a mixture of (1) an oxidant selected from the group consisting of nitric acid containing at least about 70 weight percent HNO and mixtures of said nitric acid with perchloric acid wherein said mixtures contain up to about 50 weight percent H610 and not more than about 30 weight percent water, and (2) at least one organic sulfonium compound characterized by a formula selected from the group consisting of wherein: each R is selected from the group consisting of alkyl cycloalkyl groups containing from 1 to 10 carbon atoms; R is selected from the group consisting of alkylene radicals containing from 1 to 8 carbon atoms; X is an anion selected from the group consisting of nitrate, per chlorate, sulfate, hydrogen sulfate, phosphate, hydrogen phosphate, and dihydrogen phosphate anions; a and b are each integers of from 1 to 3, and the product of a multiplied by the number of sulfur atoms is equal to the product of b multiplied by the valence z of said anion X; the total number of carbon atoms in the molecule is from 3 to 12; and wherein the ratio of said sulfonium compound to said oxidant is within the range of 0.75 to 1.25 times that of the stoichiometric amount.

2. The monopropellant of claim 1 wherein said organic sulfonium compound is trimethylsulfonium nitrate.

3. The monopropellant of claim 1 wherein said organic sulfonium compound is triethylsulfonium nitrate.

4. The monopropellant of claim 1 wherein said organic sulfonium compound is triethylsulfonium perchlorate.

5. The monopropellant of claim 1 wherein said organic sulfonium compound is tri-n-propylsulfonium perchlorate.

6. The monopropellant of claim 1 wherein said organic sulfonium compound is S,S,S',S'-tetramethylpropane-1,3-disulfonium dinitrate.

7. The monopropellant of claim 1 wherein said organic sulfonium compound is S,S,SS'-tetramethylpropane- 1,3-disulfonium diperchlorate.

8. The monopropellant of claim 1 wherein said organic sulfonium compound is S,S,S',S'-tetraethylpropane- 1,3-disulfonium dinitrate.

9. The monopropellant of claim 1 wherein said organic sulfonium compound is S,S,S',S'-tetraethylbutane- 1,4-disulfonium diperchlorate.

10. In the method for development of thrust by the combustion of a monopropellant in the combustion chamber of a reaction motor, the step comprising injecting into said combustion chamber a mixture consisting essentially of 1) an oxidant selected from the group consisting of nitric acid containing at least 70 weight percent HNO and mixtures of said nitric acid with perchloric acid wherein said mixtures contain up to about 50 weight percent HClO and not more than about 30 weight percent water, and (2) at least one organic sulfonium compound characterized by a formula selected from the group consisting of I II 7 atoms is equal to the product of b multiplied by the valence z of said anion X; the total number of carbon atoms in the molecule is from 3 to 12; and wherein the ratio of said sulfonium compound to said oxidant is within the range of 0.75 to 1.25 times that of the stoichiometric amount.

11. The method of claim 10 wherein said organic sul-' fonium compound is trimethylsulfonium nitrate.

12. The method of claim 10 wherein said organic sulfonium compound is triethylsulfonium nitrate.

13. The method of claim 10 wherein said organic sulfonium compound is triethylsulfonium perchlorate.

14. The method of claim 10 wherein said organic sulfonium compound is tri-n-propylsulfonium perchlorate.

15. The method of claim 10 wherein said organic sulfonium compound is S,S,S,S'-tetramethylpropane-1,3- disulfonium dinitrate.

16. The method of claim 10 wherein said organic sulfonium compound is S,S,S',S-tetramethylpropane-1,3- disulfonium diperchlorate.

17. The method of claim 10 wherein said organicsulfonium compound is S,S,S',S-tetraethylpropane-1,3- disulfonium dinitrate.

18. The method of claim 10 wherein said organic sulfonium compound is S,S,S,S'-tetraethylbutane-1,4- disulfonium diperchlorate.

References Cited in the file of this patent UNITED STATES PATENTS No. 72, December 1947, page 17.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3.023.569 March 6, 1962 John E. Mahan et al.

hat error appears in the above numbered pat- It is hereby certified t hat the said Letters Patent should read as ent reqiiring correction and t corrected below.

Column 6, line 15, after "alkyl" insert and line 69, for "fromt he" read from the Signed and sealed this 14th day of August 1962.

. SEAL) kttest:

DAVID L. LADD IRNEST W. SWIDER Commissioner of Patents attesting Officer 

10. IN THE METHOD FOR DEVELOPMENT OF THRUST BY THE COMBUSTION OF A MONOPROPELLANT IN THE COMBUSTION CHAMBER OF A REACTION MOTOR THE STEP COMPRISING INJECTING INTO SAID COMBUSTION CHAMBER A MIXTURE CONSISTING ESSENTIALLY OF (1) AN OXIDANT SELECTED FROM THE GROUP CONSISTING OF NITRIC ACID CONTAINING AT LEAST 70 WEIGHT PERCENT HNO3 AND MIXTURES OF SAID NITRIC ACID WITH PERCHLORIC ACID WHEREIN SAID MIXTURES CONTAIN UP TO ABOUT 50 WEIGHT PERCENT HCIO4 AND NOT MORE THAN ABOUT 30 WEIGHT PERCENT WATER, AND (2) AT LEAST ONE ORGANIC SULFONIUM COMPOUND CHARACTERIZED BY A FORMULA SELECTED FROM THE GROUP CONSISTING OF 